Websites tend to take much smaller pictures than print or default camera settings. As a result you will likely need to resize your photos. This article explains how to resize a photo for your website.
Click on "Quick Resize" to bring up the resize info box. Fill in the "Custom size" as specified by your web designer.
Choose the image you want resized from your computer. Click Browse then find the image on your computer. Click "I'm done, resize my picture".
Click "Save to disk"
That's it, now you can upload the image you just saved to your website. Alternatively there is another free online resizer called Web Resizer.